Every lens system introduces various types of optical artifacts, and the present disclosure relates to geometric distortions or rather the determination and utilization thereof.
A common type of geometric distortion in an imaging system including a zoom lens is barrel distortion. Barrel distortion manifests itself in that the magnification decreases with distance from the optical axis, barrel distortion is categorized as a radial distortion, as is pincushion distortion and moustache distortion. The effect of barrel distortion may be that a rectangular object with four straight edges as imaged will obtain a barrel-like shape where the edges are convex, hence the name.
Depending on the specification for the lens and the zoom settings the geometric distortion may be more or less pronounced. The aperture of the camera affects the quality of the image and the optimum aperture is depending on the focal length. In order to have a zoom lens operate at the sweet spot where the imaging system is optimized on image quality the aperture has to be adjusted according to the present focal length, and consequently continuously updated information on the present focal setting is of value. Some lenses uses feedback from zoom motors in order to keep track of the present focal length. However, there are also zoom lenses that do not have this feature, e.g. lenses where the zoom settings are made manually and other zoom lenses where there is no feedback function. The category “zoom lenses” is typically divided into varifocal lenses and parfocal lenses and for the purposes of the present disclosure the word “zoom lens” will mainly be used and zoom setting and focus setting or focal length setting will be used in parallel.